Nightly hanging test env precompilation with `JULIA_NUM_THREADS > 1`

[IanButterworth]

ProgressMeter.jl precompilation in the Pkg.test environment (which usually takes 1s) is hanging on nightly in CI when JULIA_NUM_THREADS=2 but not when =1.

1.6, 1.10 and 1.11 all succeed. And precompilation in the regular REPL succeeds.

https://github.com/timholy/ProgressMeter.jl/actions/runs/8127876000/job/22213188493?pr=305

I suspect that JuliaLang/Pkg.jl#3792 is the cause, but I'm not sure why (fyi @KristofferC )

[gbaraldi]

So the issue is that https://github.com/JuliaLang/Pkg.jl/blob/28bbbd46f3303920fd52e4eaccc53543f575f75b/src/Operations.jl#L2042-L2052
might pass the --threads flag to the precompile which is not recommended because precompilation with threads is not supported and may lead to fun and interesting bugs.

[gbaraldi]

This gets better with #53697, but i'd still consider what Pkg is doing a bug.

[IanButterworth]

Wasn't that all avoided with #48592 ?

[vtjnash]

It used to, but Pkg now includes code to forcibly bypass that fix

[IanButterworth]

Ah, yeah this regressed in JuliaLang/Pkg.jl#3792 which forces the gen_subprocess_flags flags through the flags kwarg

https://github.com/IanButterworth/Pkg.jl/blob/28bbbd46f3303920fd52e4eaccc53543f575f75b/src/Operations.jl#L2045-L2052